The principle of operation of a conventional optical gas sensor is as below.
Generally, the light intensity is decreased or increased by diffraction, reflection, refraction and absorption of light on the optical path. As incident light passes through the optical path, a gas on the optical path absorbs the light and the initial light intensity decreases.
When the gas concentration (J) is isotropic and distributed uniformly on the optical path, and infrared light passes through the optical path (L), the final light intensity (I) can be explained by the Beer-Lambert's law, which is the function of the gas absorption coefficient (k), path length (L) and initial light intensity (IO).That is, I=Io·e−KJL(x)  Equation (1)
The Beer-Lambert's law is expressed as the above Equation (1). If the initial optical intensity (IO) and the absorption coefficient of a gas to be measured are constant, the final light intensity (I) is expressed as a function of the gas concentration (J) on the optical path and the path length (L).
If there is no gas to be measured in the above Equation (1), i.e., if J=0, the final light intensity becomes equal to the initial light intensity.That is, I=Io  Equation (2)
Hence, the difference of the light intensities between when there is no gas to be measured and when the gas concentration is J is obtained by Equation (3).ΔI=Io·(1−e−KJL(x))  Equation (3)
However, since the conventional infrared sensor outputs a voltage in proportion to the light intensity, the output of the sensor according to the existence or non-existence of a gas is expressed as Equation (4).ΔV=α·ΔI=α·Io·(1−e−KJL(x))  Equation (4)
In order to produce an optical gas sensor having a broad range of measurement from low concentration to high concentration, first, an optical cavity (or a gas chamber) having long light path (L) should be provided; second, an infrared sensor of which the lowest limit of the detectable light intensity (Ith) is sufficiently low should be used; or third, an infrared sensor having a saturation light intensity (Isat) which is relatively high and slightly smaller than the initial light intensity (IO) radiated from an infrared source.
However, since the commercially available infrared detecting sensors (e.g., Thermopile IR sensor or Passive IR sensor) are not enough to satisfy all of the above conditions, an advantageous method of providing an optical cavity having long path is being required.
Various methods for extending the light path within a limited optical cavity have been suggested, one of which is U.S. Pat. No. 5,341,214 titled “NDIR GAS ANALYSIS USING SPECTRAL RATIONING TECHNIQUE” invented by Jacob Y. Wong. As illustrated in FIG. 1, the invention intends to provide an optical path tube structure that causes multiple reflections that result in the average path length being even greater than the physical length of the optical waveguide. Also, it intends to increase the optical path by orienting the infrared emitted from the optical source to an arbitrary direction. However, an infrared gas sensor generally has a limited field of view for receiving incident light. Due to the limited field of view, the amount of light that substantially reaches the infrared sensor for measurement is very small. Hence, the efficiency of the gas chamber is low, and the practicality lacks.
There is another method, which uses the White's Cell principle, disclosed in U.S. Pat. No. 5,009,493 titled “MIRROR ARRANGEMENT FOR A BEAM PATH IN A MULTIPLE-REFLECTION MEASURING CELL.” As illustrated in FIG. 2, a plurality of focuses lie on the reflection surfaces of mirrors, so that incident light can be reflected a predetermined number of time by the three arranged reflective mirrors, and the length of optical path can extend to analyze even a small amount of gases on the optical path.
However, since this kind of system uses a laser as source of light, it is not appropriate to measure gases like CO2. Furthermore, it is difficult to be employed in a small gas detector due to the long distance between the reflection surfaces.
Still another method was proposed by Christopher R. Sweet in U.S. Pat. No. 5,488,227 titled “GAS ANALYZER”, which constitutes a gas sensor by combination of a convex reflective mirror and a concave reflective mirror. In order to ensure an effectively long optical path, this method is characterized by installing a moving convex reflective mirror in a gas cell, as illustrated in FIG. 3. The gas analyzer according to this method comprises a structure (12) for ensuring a certain space within a gas sensor and preventing internal pollution, a cover (13), a cylindrical optical reflective mirror (15), a step motor (16) for rotating the mirror, an infrared sensor (24), a rotational disc (21) having a plurality of filters and a step motor (23) for rotating the disc.
However, since it is difficult to produce such a system and a step motor is needed for the rotation of the reflective mirror, it cannot be easily used in a small, portable and easy-to-use gas analyzer.
Still another method was disclosed in PCT/SE97/01366 (WO 98/09152) titled “GAS SENSOR” proposed by Martin. In order to provide a relatively long optical path in an optical cavity having a limited size, the method arranges three concave mirror surfaces as illustrated in FIG. 4. In other words, the gas sensor proposed by Martin comprises three elliptical concave surfaces, and it has an optical gas sensor cell structure, employing the White's cell concept of setting the focus of reflected light from each concave surface on or adjacent to the opposite reflection surface.
However, this gas sensor cell having three reflection surfaces is complex. Also, since the incident light, which is radiated from an optical source located on the surface of a main mirror (a mirror of one body) through an optical cavity, may have slight changes in its incident angle, it was difficult to determine the appropriate location of optical sensor.
The present invention relates to an optical gas sensor, more specifically to a non-dispersive infrared (NDIR) gas sensor.
There are two ways of measuring CO2 concentration. One is NDIR system, and the other is solid electrolyte system as disclosed, for example, in “A carbon dioxide gas sensor based on solid electrolyte for air quality control” in Sensors and Actuators B. vol. 66, pp. 55-66, 2000 by K. Kaneyasu, et al.
Although the solid electrolyte sensor is less expensive than the NDIR sensor, the NDIR sensor is preferable in terms of long-term stability, high accuracy and low power consumption, etc. Also, the NDIR sensor has good selectivity and sensitivity since it employs the physical sensing principle that an objective gas absorbs infrared of a certain wavelength.
The optical characteristics of the NDIR sensor are as follows.
Generally, the light intensity is decreased or increased by diffraction, reflection, refraction and absorption of light on the optical path. As for an NDIR sensor, as the incident light passes through the optical path, a gas on the optical path absorbs it and the initial light intensity becomes decreased.
When the gas concentration (J) is isotropic and distributed uniformly on the optical path, and infrared light passes through the optical path (L), the final light intensity (I) can be explained by the Beer-Lambert's law, which is the function of the gas absorption coefficient (k), path length (L) and initial light intensity (IO).That is, I=Io·e−kJL(x)  Equation (5)
The Beer-Lambert's law is expressed as the above Equation (5). If the initial optical intensity (IO) and the absorption coefficient (k) of a gas to be measured are constant, the final light intensity (I) is expressed as a function of the gas concentration (J) on the optical path and the path length (L).
If there is no gas to be measured in the above Equation (5), i.e., if J=0, the final light intensity becomes equal to the initial light intensity.That is, I=Io  Equation (6)
Hence, the difference of the light intensities between when there is no gas to be measured and when the gas concentration is J is obtained by Equation (7).ΔI=Io·(1−ekJL(x))  Equation (7)
However, since the conventional infrared sensor outputs a voltage in proportion to the light intensity, the output of the sensor according to the existence or non-existence of a gas is expressed as Equation (8).ΔV=α·ΔI=α·[Io·(1−e−kJL(x))]  Equation (8)where, α is a proportional constant.
In order to produce an optical gas sensor having a broad range of measurement from low concentration to high concentration, first, an optical cavity (or a gas chamber) having long path (L) should be provided; second, an infrared sensor of which the lowest limit of the detectable light intensity (Ith) is sufficiently low should be used; and third, an infrared sensor having a saturation light intensity (Isat) which is relatively high and slightly smaller than the initial light intensity (IO) radiated from an infrared source.
However, the commercially available infrared detecting sensors (e.g., Thermopile IR sensor or Passive IR sensor) are not enough to satisfy all of the above conditions, an advantageous method of providing an optical cavity having long path is required.
There are four kinds of optical cavities that have been applied to existing NDIR gas sensor systems.
First, as disclosed in U.S. Pat. No. 5,444,249 of Jacob Y. Wong, which was issued on Aug. 22, 1995, there is a square type or a cylindrical tube type having one infrared (IR) source and one light detector.
Next, as disclosed in U.S. Pat. No. 6,067,840 invented by Mahesan Chelvayohan and issued on May 30, 2000 or as disclosed in the Article titled “An implementation of NDIR type CO2 gas sample chamber and measuring hardware for capnograph system in consideration of time response characteristics” in Journal of Korean Sensor Society, vol. 5, no. 5, pp. 279-285, 2001 by I. Y. Park, et al., there is a type comprising one light detector and two IR optical sources for thermal aging compensation.
Third, what is disclosed in the Article titled “CO2/H2O Gas Sensor Using Tunable Fabry-Perot Filter with Wide Wavelength Range” in the IEEE International Conference on MEMS, pp. 319-322, 2003 by Makoto Noro, et al. is a type using a cylindrical tube optical cavity and applying a Fabry-Perot filter for selecting target gas wavelength.
Fourth, what is disclosed in PCT/SE97/01366 (WO 98/09152) titled “Gas Sensor” dated Mar. 5, 1998 by Martin Hans, et al. is a type comprising three concave mirrors in order to increase the light path within a chamber of a small volume.
Particularly, the method proposed by Martin relates to an optical gas sensor cell structure comprising three concave reflection surfaces and applying the White's cell concept of setting the focus of reflected light on or adjacent to the opposite reflection surface. This method has an advantage of simply providing a relatively long optical path compared with other methods.
However, since the incident light, which is radiated from an optical source located on the surface of a main mirror (a mirror of one body) through an optical cavity, may have slight changes in its incident angle, it was difficult to determine the appropriate location of the optical sensor.